G57 would be July, 1957. The N6 is their designation for the location of the factory that actually made the plate.
Dating any Homer Laughlin plate is kinda easy actually. The first letter is for the month - "A" being January and "L" is for December. Then the next two numbers are the year.
Dating a Vernon Kilns plate is a little harder, but its pretty safe to say that most of their Souvenir Plates were made in the '40s to early '50's.
Do you want to see some of my plates from Reno?